Generally, in a compact disc player (to be called "CDP" below), when a pickup of a CDP moves its position in response to driving a sled motor, if the pickup is to be maintained at a constant line velocity regardless of its displacing position, an angular velocity of the spindle motor which rotates the disc has to be controlled.
As shown in FIG. 1, conventionally, gain of the spindle motor is maintained constantly, so that the speed of the spindle motor can be linearly controlled to maintain a variation magnitude (.DELTA.W) of the angular velocity constantly for a same time period.
Relation between the angular velocity W, the track type line velocity v depending on rotation of the disc and a radius r depending on a position on a track of the disc is shown below by Formula (1). ##EQU1##
The angular velocity W is inversely proportional to the radius r as shown in FIG. 2B, since the line velocity v is fixed in the range of 1.2-1.4 m/s for each disc. Therefore, even for a same travel distance, the variation magnitude .DELTA.W of the angular velocity changes non-linearly depending on the position of the pickup.
Accordingly, if the control gain of the spindle motor is linearly constant, the angular velocity of the spindle motor to be controlled by the gain shows a variation relative to an actual angular velocity required for the relevant distance, as shown in FIG. 1.
Therefore, after a displacement of the pickup, the rotational line velocity of the spindle motor departs from the lock range of PLL (Phase Lock Loop) for controlling the CLV (Constant Linear Velocity), with the result that the data cannot be read out accurately, thereby making the system unreliable.